2Now Something Different 1st part of the course: Application Oriented2nd part of the course: Systems OrientedWhat is “Systems”?A: Not ProgrammingNot programming big things..Systems = Efficient and safe use of limited resources (e.g., disks)Efficient: resources should be shared, utilized as much as possibleSafe: sharing should not corrupt work of individual jobs
4The systems side of Databases What will we talk about?1. Data Organization: physical storage strategies to support efficient updates, retrieval2. Data retrieval: auxiliary data structures to enable efficient retrieval. Techniques for processing queries to ensure efficient retrieval3. Data Integrity: techniques for implementing Xtions, to ensure safe concurrent access to data. Ensuring data is safe in the presence of system crashes.
5Data Organization Key points 1. Storage Media “Memory hierarchy”Efficient/reliable transfer of data between disks and main memoryHardware techniques (RAID disks)Software techniques (Buffer mgmt)2. Storage strategies for relations-file organizationRepresentation of tuples on disksStorage of tuples in pages, clustering.
7Storage Media: Players Cache – fastest and most costly form of storage; volatile; managed by the computer system hardware.Main memory:fast access (10s to 100s of nanoseconds; 1 nanosecond = 10–9 seconds)generally too small (or too expensive) to store the entire databaseVolatile — contents of main memory are usually lost if a power failure or system crash occurs.But… CPU operates only on data in main memory
8Storage Media: Players DiskPrimary medium for the long-term storage of data; typically stores entire database.random-access – possible to read data on disk in any order, unlike magnetic tapeNon-volatile: data survive a power failure or a system crash, disk failure less likely than themNew technology: Solid State Disks and Flash disks
9Storage Media: Players Optical storagenon-volatile, data is read optically from a spinning disk using a laserCD-ROM (640 MB) and DVD (4.7 to 17 GB) most popular formsWrite-one, read-many (WORM) optical disks used for archival storage (CD-R and DVD-R)Multiple write versions also available (CD-RW, DVD-RW, and DVD-RAM)Reads and writes are slower than with magnetic diskTapesSequential access (very slow)Cheap, high capacity
10Memory Hierarchy cache Main memory V Lower price Higher speed disk NV Optical storageTapesTraveling the hierarchy:1. speed ( higher=faster)2. cost (lower=cheaper)3. volatility (between MM and Disk)4. Data transfer (Main memory the “hub”)5. Storage classes (P=primary, S=secondary,T=tertiary)
11Memory Hierarchy Data transfers cache – mm : OS/hardware controlled mm – disk : <- reads, -> writes controlled by DBMSdisk – CD-Rom or DVDdisk – TapesBackups (off-line)
12Main memory Disk Data Xfers Concerns:1. Efficiency (speed)can be improved by...a. improving raw data transfer speedb. avoiding untimely data transferc. avoiding unnecessary data transfer2. Safety (reliability, availability)a. storing data redundantly
13Main memory Disk Data Xfers Achieving efficiency:1. Improve Raw data Xfer speed1. Faster Disks2. Parallelization (RAID)2. Avoiding untimely data xfers1. Disk scheduling2. Batching3. Avoiding unnecessary data xfers1. Buffer Management2. Good file organization
15Surface of platter divided into circular tracks Read-write headPositioned very close to the platter surface (almost touching it)Surface of platter divided into circular tracksEach track is divided into sectors.A sector is the smallest unit of data that can be read or written.To read/write a sectordisk arm swings to position head on right trackplatter spins continually; data is read/written as sector passes under headBlock: a sequence of sectorsCylinder i consists of ith track of all the platters
17Performance Measures of Disks Measuring Disk SpeedAccess time – consists of:Seek time – time it takes to reposition the arm over the correct track.(Rotational) latency time – time it takes for the sector to be accessed to appear under the head.Data-transfer rate – the rate at which data can be retrieved from or stored to the disk.Analogy to taking a bus:1. Seek time: time to get to bus stop2. Latency time; time spent waiting at bus stop3. Data transfer time: time spent riding the bus
18Example ST3120022A : Barracuda 7200.7 Capacity:120 GB Interface: Ultra ATA/100 RPM: 7200 RPM Seek time: 8.5 ms avgLatency time?:7200/60 = 120 rotations/sec1 rotation in 8.3 ms => So, Av. Latency = 4.16 ms
19Random vs sequential i/o Ex: 1 KB BlockRandom I/O: 15 ms.Sequential I/O: 1 ms.Rule of Random I/O: Expensive Thumb Sequential I/O: Much less ~10-20 times
20Performance Measures (Cont.) Mean time to failure (MTTF) – the average time the disk is expected to run continuously without any failure.Typically 5 to 10 yearsProbability of failure of new disks is quite low, corresponding to a “theoretical MTTF” of 30,000 to 1,200,000 hours for a new diskE.g., an MTTF of 1,200,000 hours for a new disk means that given 1000 relatively new disks, on an average one will fail every 1200 hoursMTTF decreases as disk ages
21RAID RAID: Redundant Arrays of Independent (Inexpensive) Disks disk organization techniques that manage a large numbers of disks, providing a view of a single diskIdea: cheaper to have many small disks, than few big disksbonus: also advantageous for:1. speed (efficiency)2. reliability (safety)
22Improvement in Performance via Parallelism Choices:D D D Dn1. Distribute files (f1 D1, f2 D2, ....)or2. Distribute parts of files (“striping”) block striping sector striping...... bit striping
23Parallelization File distribution Striping +: improved ||’ism (speed) +: Availability: Many files still available if a disk goes downrecovery requires fewer disks- : but still sequential read for each fileStriping+: improved ||’ism (speed)( - : but a single disk failure catastrophic!)
24Improving Reliability Measure: MTTFStriping reduces reliability: why?Solution = RedundancyRedundancy: store data on more than 1 diskE.g. “mirroring” (duplicate disks) (1 disk stored on 2)Then, MTTF for both disks: 57,000 yrs! assuming MTTF foreach disk is 11 yrs.logical disk
25RAID LevelsSchemes to provide redundancy at lower cost by using disk striping combined with parity bitsDifferent RAID organizations, or RAID levels, have differing cost, performance and reliability characteristicsRAID Level 0: Block striping; non-redundant.Used in high-performance applications where data loss is not critical.RAID Level 1: Mirrored disks with block stripingOffers good write performance.Popular for applications such as storing log files in a database system.
26RAID Levels (Cont.)RAID Level 2: Memory-Style Error-Correcting-Codes (ECC) with bit striping.RAID Level 3: Bit-Interleaved Paritya single parity bit is enough for error correction, not just detection, since we know which disk has failedWhen writing data, corresponding parity bits must also be computed and written to a parity bit diskTo recover data in a damaged disk, compute XOR of bits from other disks (including parity bit disk)
27RAID Levels (Cont.) RAID Level 3 (Cont.) Faster data transfer than with a single disk, but fewer I/Os per second since every disk has to participate in every I/O.Subsumes Level 2 (provides all its benefits, at lower cost).RAID Level 4: Block-Interleaved Parity; uses block-level striping, and keeps a parity block on a separate disk for corresponding blocks from N other disks.When writing data block, corresponding block of parity bits must also be computed and written to parity diskTo find value of a damaged block, compute XOR of bits from corresponding blocks (including parity block) from other disks.
28RAID Levels (Cont.) RAID Level 4 (Cont.) Provides higher I/O rates for independent block reads than Level 3Provides high transfer rates for reads of multiple blocks than no-stripingBefore writing a block, parity data must be computedCan be done by using old parity block, old value of current block and new value of current block (2 block reads + 2 block writes)Parity block becomes a bottleneck for independent block writes since every block write also writes to parity disk
29RAID Levels (Cont.)RAID Level 5: Block-Interleaved Distributed Parity; partitions data and parity among all N + 1 disks, rather than storing data in N disks and parity in 1 disk.E.g., with 5 disks, parity block for nth set of blocks is stored on disk (n mod 5) + 1, with the data blocks stored on the other 4 disks.
30RAID Levels (Cont.) RAID Level 5 (Cont.) Higher I/O rates than Level 4.Block writes occur in parallel if the blocks and their parity blocks are on different disks.Subsumes Level 4: provides same benefits, but avoids bottleneck of parity disk.RAID Level 6: P+Q Redundancy scheme; similar to Level 5, but stores extra redundant information to guard against multiple disk failures.Better reliability than Level 5 at a higher cost; not used as widely.
31Choice of RAID Level Factors in choosing RAID level Monetary costPerformance: Number of I/O operations per second, and bandwidth during normal operationPerformance during failurePerformance during rebuild of failed diskIncluding time taken to rebuild failed diskRAID 0 is used only when data safety is not importantE.g. data can be recovered quickly from other sourcesLevel 2 and 4 never used since they are subsumed by 3 and 5Level 3 is not used anymore since bit-striping forces single block reads to access all disks, wasting disk arm movement, which block striping (level 5) avoidsLevel 6 is rarely used since levels 1 and 5 offer adequate safety for almost all applicationsSo competition is between 1 and 5 only